PTL 1 discloses a method of measuring specific humidity of air by using a sensor means constituted by a capacitive sensor, a temperature sensor, and a heating element, providing a predetermined humidity limit value, and changing temperature of the capacitive sensor when reaching the predetermined humidity limit value. In the method, a capacity of the capacitive sensor for obtaining a certain capacity value, humidity of air until reaching the predetermined humidity limit value is measured by evaluating the capacity value, the capacitive sensor is heated to adjust temperature of the sensor means such that the capacitive sensor is adjusted to a constant capacity when reaching the predetermined humidity limit value, and the temperature is evaluated. According to PTL 1, it is possible to stably and accurately measure the specific humidity for a long period of time even in the vicinity of a dew point without causing a measurement result to be affected by an error nor causing a drift in a sensor signal even if being used for a long period of time in a high-humidity range, and thus, it is also possible to stably measure relative humidity for a long period of time near 100% of relative humidity.